Cell walls of all plants have, as basic skeletons, high-strength nanofibers having a width of about 4 nm referred to as microfibrillated cellulose. Microfibrillated cellulose is nanofibers constituted by extended-chain crystals and obtained by fibrillating vegetable fibers such as pulp to the level of microfibrillated cellulose. In addition, microfibrillated cellulose derived from bacteria (mainly, Acetobacter) is also known; and a food employing this microfibrillated cellulose is nata de coco, which is well known. It is known that microfibrillated cellulose can be generally produced by milling or beating cellulose fibers with a refiner, homogenizer, or the like (for example, refer to Patent Literature 1). Microfibrillated cellulose is lightweight and has high strength and high biodegradability, and hence is expected to be applied to wide-ranging fields: for example, casings of household electrical appliances such as personal computers and cellular phones, office equipment such as writing materials, sports goods, transport equipment, and building materials.
In recent years, a microfibrillated cellulose that can be dispersed in an organic solvent by surface modification of the cellulose with a monoisocyanate has been known (Patent Literature 2).
Although such modified microfibrillated cellulose is dispersed in an organic solvent, it still has hydrophilicity due to remaining of hydroxyl groups on the surface and hence it is not satisfactory. The modified microfibrillated cellulose has the following problems: when the modified microfibrillated cellulose is added to a resin, the phenomenon of agglomeration and aggregation occurs; and, even when the modified microfibrillated cellulose is mixed with a resin, the mechanical properties of the resin are not enhanced.
A method has been studied in which microfibrillated cellulose is surface-treated with a silane coupling agent and then combined with a resin to produce a composite resin to thereby provide the composite resin having enhanced mechanical properties (Patent Literature 3). However, in such a case, since the surface-treatment agent is a silane coupling agent, which is a low-molecular-weight monomer, a large amount of the silane coupling agent needs to be bonded to the surface of microfibrillated cellulose so that the silane coupling agent exhibits its capability, that is, a composite resin having enhanced mechanical properties is provided. When bonding of such a larger amount of the silane coupling agent to the surface of microfibrillated cellulose is attempted, polycondensation between molecules of the silane coupling agent, which is a side reaction, occurs during the surface treatment and the treatment cannot be efficiently performed, which has been problematic.